This article proposes a model to explain electoral reforms, understood as competitive political processes that evolve in a democratic context. The model departs from a political crisis context expressed in a strong electoral volatility that leads to two different extreme types of electoral reform. When the reform is promoted by a "declining coalition", legitimacy arguments are mobilized and more inclusive rules are proposed. When the promoter is a "growing coalition", arguments on efficacy are used and more exclusive rules are put forward. The model is supported by an empirical analysis of all the reforms affecting presidential electoral rules that took place in South America from 1990.